[fpc-devel] Crash in TWriter with read only class property = nil

[Martin Frb]

If you have a readonly property such as
(tested with 3.2.2 rc)

   TMyComponent = class(TComponent)
   private
     FFoo: TComponent;
   published
     property ReadOnly: TComponent read FFoo;
   end;

and FFoo is nil,
then TWriter will crash.

Is the above scenario forbidden to be implemented (and therefore the 
crash a result of breaking some rule)
or should that work (and the crash is a bug)?

 From 3.2.2 rc sources:

procedure TWriter.WriteProperty(Instance: TPersistent; PropInfo: Pointer);
begin
   // do not stream properties without getter
   if not Assigned(PPropInfo(PropInfo)^.GetProc) then
     exit;
   // properties without setter are only allowed, if they are subcomponents
   PropType := PPropInfo(PropInfo)^.PropType;
   if not Assigned(PPropInfo(PropInfo)^.SetProc) then begin
     if PropType^.Kind<>tkClass then
       exit;
     ObjValue := TObject(GetObjectProp(Instance, PropInfo));
     if not ObjValue.InheritsFrom(TComponent) or   // 
<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
        not (csSubComponent in TComponent(ObjValue).ComponentStyle) then
       exit;
   end;

The marked line calls InheritsFrom on a nil value.


#0 TWRITER.WRITEPROPERTY(TWRITER($0000000000145A60), 
TPERSISTENT($000000000013A970), POINTER($0000000100378B08)) at 
c:\FPC\fpc_3.2.2\source\rtl\objpas\classes\writer.inc:895
#1 TWRITER.WRITEPROPERTIES(TWRITER($0000000000145A60), 
TPERSISTENT($000000000013A970)) at 
c:\FPC\fpc_3.2.2\source\rtl\objpas\classes\writer.inc:850
#2 TWRITER.WRITECOMPONENTDATA(TWRITER($0000000000145A60), 
TCOMPONENT($000000000013A970)) at 
c:\FPC\fpc_3.2.2\source\rtl\objpas\classes\writer.inc:764
#3 TCOMPONENT.WRITESTATE(TCOMPONENT($000000000013A970), 
TWRITER($0000000000145A60)) at 
c:\FPC\fpc_3.2.2\source\rtl\objpas\classes\compon.inc:454
#4 TWRITER.WRITECOMPONENT(TWRITER($0000000000145A60), 
TCOMPONENT($000000000013A970)) at 
c:\FPC\fpc_3.2.2\source\rtl\objpas\classes\writer.inc:688
#5 TWRITER.WRITEDESCENDENT(TWRITER($0000000000145A60), 
TCOMPONENT($000000000013A970), nil) at 
c:\FPC\fpc_3.2.2\source\rtl\objpas\classes\writer.inc:778
#6 TWRITER.WRITEROOTCOMPONENT(TWRITER($0000000000145A60), 
TCOMPONENT($000000000013A970)) at 
c:\FPC\fpc_3.2.2\source\rtl\objpas\classes\writer.inc:1278